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I have Chlorine and Algae

My wife just called me at work and said she saw a bunch of algae in the pool. She brushed it, but I'm perplexed. I've suspected some algae over the last few weeks, but haven't been sure if it's dirt or algae. Having a blue/green pebble finish doesn't make it as easy to discern algae.

What has me perplexed is that FC has been 3.5 - 4.0 every time we've tested it (which is usually 3 times a week unless something seems wrong). When we've had algae problems in the past, they've been preceded by a sudden drop in FC. I've seen the pattern and know to go into shock mode. But that's not the pattern I'm seeing this time.

Thanks to TFP, I'm pretty successful at shocking the pool and getting things back in order. What I'm trying to figure out now is what's causing the algae when everything looks good?

I'm going from memory of my test on Saturday, but I believe these were the values on Saturday morning:

Temp 74 F (this is early in the morning; it will get to the high 70s in the afternoon)[/*:m:3j82blcq]

FC 4.0[/*:m:3j82blcq]

CC 0.0[/*:m:3j82blcq]

pH 7.6 (it's usually 7.5 and the auto pH system was adding some acid on Saturday, so I assume it's 7.5 again)[/*:m:3j82blcq]

TA 60 (it's usually been 70; I had another test a few weeks ago that was down to 60, but then it went back to 70 so I didn't do anything)[/*:m:3j82blcq]

CH 450 (I just have hard water; I replaced about 50% of the water last year to try lowering CH, but it's back)[/*:m:3j82blcq]

CYA 90 (Higher than I want. I added some stabilizer about 3 weeks ago and by the measurements, I didn't expect it to get as high as it did. I thought I'd end up around 70. This surprised me. Could this be related to algae? If nothing else, it's going to take more chlorine to shock it.)[/*:m:3j82blcq]

I brushed the pool on Saturday morning and didn't notice much of anything cloudy.

A couple of weeks ago I suspected algae, as I brushed the pool and stuff was coming up. It was hard to tell if it was algae or if I got some dirt from winds (I live near a ball field which is sometimes a source of dirt in my pool). I brushed it and it cleared up and I didn't really notice much more after that, so I concluded it was just dirt, especially since FC was pretty constant.

My only thought is looking at the Shock/CYA Chart is that target FC is higher with the CYA up where it is. Do I just need to shock it and keep my FC level higher because my CYA is high?

Re: I have Chlorine and Algae

Algae sometimes forgets to follow our rules here.

It hides in poor circulation areas and develops bio films that help it hide from it's enemies.

So, while your VERY high CYA may be a contributor, the answer for algae in a pool is always the same....begin the shock process. And, yes, you will need a ton of chlorine to be succesful....don't try to do it with your SWG.

Re: I have Chlorine and Algae

Your CYA is 90 ppm but your FC has been 3.5 or 4 every time you've tested it. The minimum FC in an SWG pool with a CYA of 90 is 4 ppm or perhaps 4.5 ppm and that would be at the lowest point in the day so in the morning before the pump and SWG come back on. There's no mystery here. The minimum FC level, especially for SWG, is pretty absolute since there's not much leeway since we set the level as low as possible given the automated dosing of the SWG.

I understand that you probably thought 3.5 or 4 was OK because you thought you had 70 ppm CYA, but apparently you did not and that little bit makes a difference. You are right that you were slowly developing algae in the pool and it really didn't appear overnight, but it seems that way because it wasn't really obvious as it was growing in the previous days.

Anyway, you know how to shock the pool, but might consider lowering the CYA some first if you plan to do that anyway. Well, compromise and elevate the chlorine at least some to prevent the algae from growing a lot (say, 20 ppm), but do a partial drain/refill. Then with the CYA lower, you can do a full shock FC. Up to you -- if you don't mind using more chlorine you can go for it now.

Re: I have Chlorine and Algae

Thanks everyone. I had enough chlorine on hand last night to get FC to 18 and I tested in the morning and it was still 18 (and I turned off the SWG so it wasn't generating overnight). Does that mean I killed it all? Or are biofilms mentioned by duraleigh keeping my enemy alive? We're brushing the pool. Is a brush enough to break down the biofilms? Or is it the high FC that breaks through? Looking at the chart, I need to shock at 35.

For the moment I'm opting not to drain to lower CYA and just plan to shock at real high FC. Historically, my CYA tends to go down faster than it seems it should from what I read at TFP, so I'm hoping after a few weeks it might be lower. Maybe sometime I'll start a thread on my CYA curiosities.

Re: I have Chlorine and Algae

If your chlorine held overnight (<= 1 ppm FC drop) and you have minimal CC (<= 0.5 ppm) and the water is crystal clear, then you caught this early so it cleared quickly. For safety, I'd still raise the FC to at least 30 ppm for 24 hours, then let it drop down to a minimum of no lower than 4 ppm (in the morning just before the pump and SWG turn on) though why don't you shoot for 5 ppm FC just to be safe. If your CYA drops over time to 70 ppm, then you can lower your target to 3 ppm as an absolute minimum (per the chart in the Pool School), though again you could target 4 ppm to be safe. Remember that your pool is clearly "on the edge".

Re: I have Chlorine and Algae

Thanks chem geek and others. I'll dump in more chlorine to get it up to at least 30.

How low does FC need to get to be "safe" to swim? Usually with my CYA lower, when I've had to shock, I'm targeting FC in the 20s, but I found it doesn't drop quickly at all once I've killed everything. I've resorted to chlorine neutralizer (expensive stuff) to get FC down, as I've waited over a week when FC's in the mid 20s and it might still be around 18 or so.

Looking at chem geek's Chlorine/CYA chart, I see for CYA 90 that the FC min for yellow/mustard algae is 13.5, so that should be safe to swim in. In that thread, you mentioned that most indoor pools have FC far higher than even shock levels, so do I even need to worry about lowering the chlorine before taking a plunge? I have children who are anxious to swim.

Re: I have Chlorine and Algae

You can swim up to the regular shock FC level. The active chlorine is roughly equivalent to 0.6 ppm FC with no CYA so somewhat less than many indoor commercial/public pools with no CYA. While the chlorine will react with skin, hair, swimsuits around 8 times faster than the minimum FC level, it's safe to swim in. The main risk is in drinking lots of pool water due to the high FC, though you'd still need to drink a lot and to do so every day before you even begin to have to worry about issues. The EPA drinking water limit is 4 ppm FC assuming one drinks 2 liters or quarts every day for a lifetime.

The FC should drop during the day if the pool gets exposed to sunlight.

Re: I have Chlorine and Algae

As a follow up, somehow my successful overnight FC loss test when I was at FC 18 was a false feeling of success. The following night, I brought it up to 40 and in the morning it had dropped to 35.5.

Now I've had two successful nights:
33.5 > 33.0
29.5 > 29.5

Finally this morning when brushing, I'm not noticing very much stirring up! It hasn't been great quantities stirring up over the last couple of days, but there's certainly been stuff. I got tired of brushing to stir up stuff and get it through the filtration system, so yesterday morning I vacuumed.

I think tomorrow I'll backwash the filter and it looks like I'm in good shape!

Thanks everyone for your help. I got to learn new things with this problem!